The present specification relates generally to the field of displays. More particularly, the present specification relates to a method of and an apparatus for utilizing a tunable filter in a display.
In general, it is desirous to utilize filters in display technology to attenuate or accentuate particular types of electromagnetic radiation. For example, certain displays and visual equipment may desire to accentuate particular colors in the visible light spectrum and attenuate other colors in the non-visible and the visible light spectrum. Accordingly, these display can utilize filters to provide accentuation and attenuation.
In one particular application, displays and other equipment utilized in military, sports and transportation activities are often employed in tandem with night vision equipment. These displays and equipment conventionally utilize a filter to accommodate the night vision equipment. Issues related to the use of displays and night vision equipment are described below with reference to an aviation application, although the below-mentioned issues are relevant to any applications of displays requiring attenuation or accentuation.
Certain aviation displays are color displays that are utilized with night vision imaging systems (NVIS). These displays provide visual information to captains, pilots, and drivers of ships, aircraft, and vehicles. The user of the color display often wears NVIS goggles at the same time he or she observes information from the color display.
Conventional NVIS goggles are sensitive to light in the infrared, near infrared and visible red spectrum (wavelengths of light). NVIS goggles are typically sensitive to light between 600 nm and 950 nm wavelengths. Although the NVIS goggles allow the pilot or person wearing the goggles to see objects which cannot ordinarily be seen by the naked eye, the goggles bloom (emit bright light) if cockpit lighting is too intense in the spectral region where the goggles are sensitive. The net result, which is highly undesirable, is a loss of contrast when the pilot is looking through the goggles.
Additionally, when the goggles emit the bright light, the pilot""s eyes may lose their night adaptation (e.g., night vision). Restoring full night adaptation can take several minutes. Accordingly, the bloom effect is undesirable when operating a vehicle or aircraft in night vision conditions.
Conventional avionic displays designed to be utilized with NVIS equipment generally are restricted to a narrow emission, such as, single color (e.g., green) displays. The narrow emission is chosen so that it does not interfere with NVIS equipment. However, the restriction to the narrow emission significantly reduces the readability of information and the symbology provided on the displays. Further, it is difficult to highlight and differentiate large amounts of information on the display if the display is restricted to a single color.
Other conventional avionic systems have included color displays that include an NVIS filter. The color display operates in two modes, an NVIS mode (e.g., low luminance) and daylight mode. The NVIS filter is provided between a light source used in the NVIS mode and an optical shutter, such as a liquid crystal display. The filter prevents emissions that cause NVIS equipment to bloom.
In the daylight mode, the displays use a second light source to provide light directly through the optical shutter without traversing the filter. The second light source is positioned so that its light is not provided through the NVIS filter.
Conventional NVIS filters are generally comprised of glass or other material supplemented by thin film coatings that attenuate infrared emissions or transmissions. Conventional NVIS filters are generally relatively imprecise at its cutoff frequency. This characteristic is particularly problematic because the frequency at which NVIS goggles are sensitive is extremely close to the frequency at which red emissions exist. Accordingly, a precise cutoff frequency is needed in NVIS filters so that red colors can be effectively utilized on a display.
With reference to FIG. 1, the transmittance of a sample of filters is shown with respect to wavelength. As can be seen in FIG. 1, four different filters manufactured from the same material can have significantly different cutoff frequencies as represented by graphs 10, 12, 14 and 16. The difference in cutoff frequency is due to the tolerances associated with the manufacture of the materials and the deposition of the thin films associated with the NVIS filter. For example, variations in the thickness of the thin films cause variations in the cutoff frequency associated with the filter. If the filter has a cutoff wavelength that is too small, the filter attenuates visible color in the red range and the pilot is not able to view red colors on the display. If the cutoff wavelength is too large, the NVIS goggles receive emissions in the infrared and near infrared range and are susceptible to bloom effects. Manufacturing NVIS filters with tight tolerances is expensive and technologically challenging. Typically available NVIS filters have cutoff wavelengths varying from 600 to 640 nm.
Thus, there is a need for a display system that can utilize inexpensive NVIS filters. Further, there is a need for a display system which utilizes a tunable NVIS filter. Further still, there is a need for a system which can utilize a filter having a cutoff frequency within a large tolerance. Further still, there is a need for an avionic display which can utilize an inexpensive NVIS filter.
An exemplary embodiment relates to a display including a light source. The display includes a filter positioned to receive light from the light source. The filter has a different wavelength characteristic depending upon an angle of incidence of the light from the light source. The angle of incidence of the light is adjustable.
Yet another exemplary embodiment relates to an avionic display system. The avionic display system includes an optical shutter, a light guide, a first light source and a night vision filter. The night vision filter is positioned to receive light from the first light source. The filter has a different wavelength characteristic depending upon an angle of incidence of the light from the first light source. The angle of incidence of the light is adjustable. The light travels through the filter to the light guide to the optical shutter.
Yet another embodiment relates to a method of calibrating a filter for use in a display system. The display system includes a light source and the filter. The angle of incidence of light on the filter is adjustable. The filter has a cutoff characteristic varying according to the angle of incidence of light from the light source. The method includes measuring at least a portion of the spectrum of light provided through the filter, and adjusting the angle of incidence of light on the filter until a desired cutoff characteristic is achieved.
Yet still another exemplary embodiment relates to an avionic display system. The avionic display system also includes a means for providing a visual image, and a night vision filter means for attenuating light associated with the visual image above a selectable wavelength.